The requirements for the communication of information have rapidly been increased along with the recent wide spread of the internet. For this reason, there has widely been spread the wavelength division multiplexing (WDM) transmission technique in which signals are transmitted within a single optical fiber through the use of a plurality of light rays having different wavelengths. In case of the long distance communication, there has been used dense wavelength division multiplexing (DWDM) transmission technique which makes use of a large number of light rays having different wavelengths and in which the wavelength range used is reduced to a level of not more than 1 nm to thus carry information on a single optical fiber in a quantity of information as great as possible. In this case, it is essential that the fluctuation in each wavelength should be not more than 0.1 nm and it has been required that the fluctuation is preferably not more than 0.01 nm.
On the other hand, there has been spread, for the communication within the distance ranging from several to several tens of kilometers, the coarse wavelength division multiplexing (CWDM) transmission technique in which the wavelength range used is expanded to a level of not less than 20 nm in order to cope with such a fluctuation in wavelength on the order of about 10 nm. In this case, the use of a temperature control means such as a laser can be omitted and therefore, this may reduce the cost required for communication. There has been required for the use of an optical multiplexer for composing or coupling a plurality of optical signals having different wavelengths and a demultiplexer for classifying such signals into groups each having a single wavelength in order to realize such WDM transmission.
As an optical multiplexer-demultiplexer for the DWDM transmission, there has been known, for instance, one which makes use of an array diffraction grating (AWG) as disclosed in Technical Digest of Optical Fiber Conference 2001, WB1-1 (2001), but this optical multiplexer-demultiplexer can cope only with narrow wavelength fluctuations of not more than 0.1 nm and it cannot perform any optical multiplexing or coupling and demultiplexing operations when the wavelength fluctuation is on the order of 10 nm like the CWDM transmission.
As an optical multiplexer-demultiplexer which can cope with a wide wavelength fluctuation like one for the CWDM transmission, there has been reported, for instance, an optical multiplexer-demultiplexer which makes use of an optical waveguide and a diffraction grating such as that disclosed in Applied Optics, 1982, 21: 2195. This optical multiplexer-demultiplexer comprising the combination of these two components permits the reduction of the number of parts to be used and the miniaturization of this device.
Moreover, as a method for realizing the CWDM transmission, there has been known, for instance, one which comprises the step of demultiplexing light rays using a plurality of multilayered thin film filters, each of which can pass light rays of a desired wavelength therethrough, in the number corresponding to that of the wavelengths of these light rays. The optical demultiplexer used in this method comprises a wavelength filter consisting of about 100 thin layers, whose thickness is highly precisely controlled, a collimating lens and a fiber which are assembled in such a manner that the optical axes thereof are in good agreement with one another.
The conventional optical multiplexer-demultiplexer which makes use of an AWG can simply cope with the narrow wavelength fluctuation of not more than 0.1 nm and therefore, it cannot perform any optical multiplexing or coupling and demultiplexing operation when the wavelength fluctuation is on the order of 10 nm like the CWDM transmission. The optical multiplexer-demultiplexer for the CWDM transmission should satisfy the requirement for morphological characteristics such as the flat top-shaped characteristics as shown in the attached FIG. 9, in which the transmission loss does not vary even for the wavelength fluctuation of about 10 nm. However, the conventional optical multiplexer-demultiplexer which made use of a diffraction grating showed cone-shaped characteristics as disclosed in the foregoing article: Applied Optics, 1982, 21: 2195, in which the transmission loss increased in proportion to the wavelength fluctuation. Moreover, in a multiplexing and demultiplexing method using a multilayered thin film filter, it is necessary to use a plurality of multilayered thin film filters, which are quite expensive and whose mass-production is quite difficult, in proportion to the number of wavelengths and it is also needed to highly precisely position the same with respect to a precise lens and/or an optical fiber. Thus, the resulting device is quite expensive. In addition, a problem arises such that the productivity thereof is likewise low and accordingly, the mass production thereof is quite difficult.
In the structure of an optical multiplexer-demultiplexer which makes use of an optical waveguide and a diffraction grating, if the optical waveguide used has a large absolute value (hereunder referred to as “birefringence index”) of the difference between the refractive index (nTE) of the core layer of the optical waveguide in the direction parallel to the plane of the film and that (nTM) of the core layer thereof in the direction perpendicular to the plane of the film, a problem arises such that the wavelength of the outputted (or outgoing) signal light varies depending on the polarization direction of the inputted signal light. As has been described above, the optical multiplexer-demultiplexer for the CWDM transmission should perform multiplexing and demultiplexing operations even when the wavelength fluctuation in the outputted signal light is at least 10 nm. The wavelength fluctuation in the light oscillated from a laser as a light source may be at least 5 nm due to the dispersion of the laser produced and the temperature change in the environment in which the laser is used and therefore, the wavelength fluctuation due to the birefringence index should be not more than 5 nm.
If the variation in wavelength, the wavelength used, the refractive index at the wavelength used and the birefringence index at the wavelength used are defined to be Δλ(nm), λ(nm), n and Δn, respectively, the relation between the birefringence index and the wavelength fluctuation is in general expressed by the following equation:Δλ=λ×Δn/n  (1)
For instance, when forming a film of a fluorinated polyimide used as a material for optical waveguides on a silicon substrate, the refractive index n of the film is found to be 1.5291 and the birefringence index Δn thereof is found to be 0.009 at a wavelength used of 1300 nm. From the foregoing, the wavelength fluctuation Δλ is found to fall within the range of from about 7 to 8 nm and this becomes a serious obstacle in practical applications.